1. Field of the Invention
The present invention relates generally to an image forming apparatus, and a method and a device for creating dither patterns.
2. Description of the Related Art
Electrophotographic image formation is generally inferior to offset printing in image consistency. To alleviate this disadvantage, various techniques have been attempted to improve image consistency of electrophotographic image formation to near the level of offset printing.
One of differences between electrophotography and offset printing is the difference in halftoning. Electrophotography simulates a continuous-tone image by applying image processing such as dithering to image data using a dither pattern such as a line screen or a dot screen.
Meanwhile, dot screens used in electrophotography exhibit high image consistency at low and high gray levels but exhibit low image consistency at medium gray levels. Line screens exhibit high image consistency at the medium gray levels but exhibit low image consistency at the high gray levels. Hence, there are gray levels at which high image consistency is unachievable even if both a dot screen and a line screen are used.
Hybrid screening, which is one of known techniques for alleviating this problem, applies halftoning using a dot screen and a line screen while switching therebetween depending on a gray level range. For example, Japanese Laid-open Patent Application No. 2012-3180 discloses a technique that uses a line screen in a medium gray level range but uses a void-dot screen in a high gray level range to make use of the advantages of dot screens and line screens. Line screens have a disadvantage that as the gray level increases, the width of an area where no toner is deposited (i.e., area between lines) (hereinafter, “no-deposition area”) decreases, resulting in unsteady toner deposition in the no-deposition area. To overcome this disadvantage, the technique disclosed in Japanese Laid-open Patent Application No. 2012-3180 switches from the line screen to the void-dot screen, thereby increasing smoothness of tonal gradation from the medium gray levels to the high gray levels.
However, the conventional technique is disadvantageous in that image quality such as color consistency can drop in a gray level range where switching from a line screen to a void-dot screen is made. Furthermore, the conventional technique requires that the two different types of dither patterns be stored, which arises the need of increasing the capacity of memory where the dither patterns are to be stored. Furthermore, the conventional technique requires that dots in a portion where the two different types of dither patterns adjoin be rearranged on a per-dot-matrix basis, which undesirably increases time taken for image processing.
Under the circumstances, there is a need for an image forming apparatus, a dither-pattern creation device, and a dither-pattern creation method that allow enhancing image consistency in the range from medium gray levels to high gray levels and, even if a line screen and a void-dot screen are used, reducing an increase in memory capacity and preventing an increase in image processing time.